This invention relates to locating non-visible objects, particularly though not exclusively for the purpose of identifying the position of a non-visible object prior to carrying out a mechanical processing step in the vicinity of the object so located.
There are many situations where it is desired to locate something accurately, although the item in question is not visible. A simple example is to locate the position of a load-bearing member in a partition wall made of a wooden frame to either side of which sheets of plasterboard are attached. If it is desired to fix something to the wall, e.g. using a hook, it is necessary to ensure that the hook, e.g. screwed into the wall, goes into part of the timber support rather than into the plasterboard, from which it will be easily removed when a load is applied because plasterboard is not particularly strong. Conventional methods, such as tapping the wall with a knuckle to determine the location of the supporting wooden frame members do not give particularly accurate results and require skill. Making a pilot hole through the plasterboard and inserting a piece of bent wire through it into the cavity likewise is not easy to carry out simply, and although location, e.g. using a small magnet, of the usually iron nails which hold the plasterboard to the wooden structure can be employed, again the results tend to be rather inaccurate, although this last approach does have the advantage of avoiding trying to insert a hook where there is already a nail underneath. U.S. Pat. No. 5,917,314 discloses a capacitative sensing system for finding wall studs, while U.S. Pat. No. 5,434,500 describes a system for marking a position on a partition precisely opposite a selected position on the other side against which a magnetic field generator is held.
These systems are useful in the building trades, but are not adapted for use in situations where dimensions are subject to tight tolerance limits, some of which are particularly critical in manufacture. For example, in the manufacture of aircraft, a widely used technique is the application of a metal plate or skin to an underlying frame, for example made of ribs or spars. In order to ensure a firm connection between the skin and the rib or spar, a technique commonly employed is that of fastening the two together, e.g. with a rivet or special fastener. In order to do this, apertures in the skin and the rib or spar need to coincide and this coincidence needs to be particularly accurate since if there is inaccuracy, riveting may be rendered more difficult, or even impossible and inadequately-fitting or mis-applied rivets can become loosened when the aircraft is in service leading to potentially catastrophic failure. Accordingly, the requirements for accurate matching of the hole in the skin with the hole in the rib or spar are very stringent and the penalty for inadequate accuracy may well be the failure of the finished assembly to meet the required rigorous safety standards, leading to the entire assembly having to be recycled. Although if the rib or spar has pre-formed holes, it is notionally possible to use each of those holes as successive guides for making holes in an applied skin, this is usually awkward and sometimes practically impossible for reasons of space, and inaccuracies creep in. Additionally, drilling a hole through the skin from inside does not always provide accurate alignment of the hole in the skin, so that its axis runs exactly perpendicular to the surface of the skin. This is a particular problem where the skin is varying in thickness, e.g. tapering from a thick to thin section. Working from the outside, however, i.e. working with the skin between the operator and the spar or rib means that the positions of the holes cannot be seen. Attempts to use templates to overcome this have not been successful.